Recently, there is increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention.
Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, have drive voltage and energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries), and thus they are spotlighted in the field of secondary batteries. However, lithium secondary batteries have problems related to safety, caused by ignition and explosion due to the use of organic electrolytes, and are manufactured through a complicated process. Lithium ion polymer batteries, appearing more recently, solve the above-mentioned disadvantages of secondary lithium ion batteries, and thus become one of the most potent candidates of the next generation batteries. However, such secondary lithium ion polymer batteries still have low capacity compared to secondary lithium ion batteries. Particularly, they show insufficient discharge capacity at low temperature. Hence, there is an imminent need for the improvement of secondary lithium ion batteries.
Such batteries have been produced by many battery producers. However, most lithium secondary batteries have different safety characteristics depending on several factors. Evaluation of and security in safety of batteries are very important matters to be considered. Particularly, users should be protected from being injured by malfunctioning batteries. Therefore, safety of batteries is strictly restricted in terms of ignition and combustion in batteries by safety standards. Many attempts have been made to solve the problem related to the safety of a battery. However, ignition of a battery, caused by a forced internal short circuit due to external impacts (particularly, in the case of a customer-abused battery) cannot be solved yet.
More fundamentally, currently available lithium ion batteries and lithium ion polymer batteries use polyolefin-based separators in order to prevent short circuit between a cathode and an anode. However, because such polyolefin-based separators use a polymer component having a melting point of 200° C. or less and are subjected to a stretching step for controlling their pore sizes and porosities so as to be used as separators, they have a disadvantage in that they show high heat shrinking property upon exposure to high temperature. In other words, such separators can be shrunk or molten when the temperature of a battery increases due to internal and/or external factors. Therefore, there is a great possibility of short-circuit between a cathode and an anode that are in direct contact with each other due to shrinking or melting of separators, resulting in accidents such as ignition and explosion of a battery caused by rapid emission of electric energy.
To solve the above problems related with polyolefin-based separators, many attempts are made to develop an electrolyte using an inorganic material capable of substituting for a conventional separator.
U.S. Pat. No. 6,432,586 discloses a polyolefin-based separator coated with an inorganic layer, such as calcium carbonate, silica, etc. However, the separator merely using the polyolefin-based polymer cannot provide a significant improvement in the safety of a battery, particularly in terms of the prevention of heat shrinking at high temperature.
Additionally, Creavis Co. (Germany) have developed an organic/inorganic composite separator comprising a non-woven polyester support coated with silica (SiO2) or alumina (Al2O3). However, in the case of the above separator, the non-woven polyester support cannot provide excellent mechanical and physical properties by nature, and the chemical structure of polyester is liable to electrochemical reactions. Thus, it is thought that the above separator shows many problems in practical use (“Desalination”, vol. 146, p. 23 (2002)).
Accordingly, there is an imminent need for developing a separator that can improve the quality and safety of an electrochemical device, or a composite electrolyte that also serves as such a separator.